Hitherto, piping members made by molding of a propylene resin composition have been widely utilized in a variety of factories, medical fields and construction fields, for example, since the propylene resin composition can exhibit excellent properties such as rigidity, heat resistance and chemical resistance. Especially, pipes of the propylene resin are suitable for use as the piping member in the industrial fields through which a chemical or its solution at a high temperature is guided, since the propylene resin-made pipes exhibit a good chemical resistance to acids and alkalis at an area of increased temperature (60 to 95° C.), and are produced at a low price. Accordingly, there is a tendency to use resin-made pipes in place of the prior art metal-made pipes conventionally used. Further, a propylene resin-made pipe has been proposed for such resin-made pipes, and thus is expected to be widely used in the future. Furthermore, recently, there have been requested at an increasing degree of need to provide a transparent piping member capable of visually observe, from an outside of the piping member, foreign substances contained in the flowing fluid in the piping member for the purpose of improving safety of the piping system. Accordingly, ensuring transparency of the piping members made of a propylene resin which are able to be used under all the application conditions is being pursued.
In certain fields, “translucent” piping members made of a propylene resin have been used. However, when a transparency of the propylene resin is increased in such piping members, physical properties such as impact strength and tensile elasticity which are required in the piping members are reduced. Accordingly, using such piping members made of a propylene resin which have a certain level of transparency along with the physical strength suitable as the piping member, the transparency is insufficient to confirm a state of the flowing fluid in the piping member. The transparency is a level of the haze value of about 78% at which it is only able to confirm whether or not a fluid is flowing in the piping member.
As a method of improving a transparency of the piping member molded from the propylene resin composition, it is well known, for example, to add a nucleating agent for clearing to the polypropylene resin (see, Patent Reference 1). Described therein is a pipe joint for food piping which is used in the formation of piping system of foods and is constituted with a female element and a male element capable of being inserted in an inner surface of the female element. The female element has a transparent section through which at least a tip side of the inserted male element can be visually confirmed, and the transparent section is made of the propylene resin having added thereto a nucleating agent for clearing. Normally, the polypropylene resin has a color of white or creamy white, but a transparency of the pipe joint made of the same resin can be increased if a nucleating agent for clearing is added to the resin. That is, according to this method, since the female element has a transparent section formed by adding the nucleating agent to the polypropylene resin, it is possible to visually confirm at least a tip side of the male element inserted into the pipe joint.
However, when the nucleating agent is added to the polypropylene resin as is described above, there is caused a fatal problem, i.e., lowering in the impact resistance required in the piping member due to addition of the nucleating agent, while the transparency of the piping member can be improved as a result of addition of the nucleating agent. Due to lowering of the impact resistance, the pipe joint can be relatively easily cracked, and thus the applications of the pipe joint are unusually limited to, for example, use under the lowered inner pressure.
Then, in order to simultaneously improve both the transparency and the impact resistance of the polypropylene resin composition, there have been suggested a polypropylene sheet for use in a press through pack (PTP) packaging (see, Patent Reference 2). The PTP packaging is applied to the packaging of medicines such as tablets and capsules. The polypropylene sheet comprises 57 to 84% by weight of polypropylene homopolymer containing a crystal nucleating agent in an amount of 0.002 to 0.3% by weight, 10 to 25% by weight of ethylene-propylene random copolymer containing an ethylene in an amount of 0.1 to 5.0% by weight, 1 to 8% by weight of a petroleum resin and 5 to 10% by weight of a thermoplastic elastomer. The thermoplastic elastomer added is a hydrogenated styrene-butadiene copolymer containing a styrene in an amount of 11 to 35% by weight or a hydrogenated styrene-isoprene copolymer containing a styrene in an amount of 11 to 35% by weight. The propylene sheet has excellent properties such as rigidity, moisture resistance, transparency, moldability and impact resistance.
The prior art resin composition described above can improve impact resistance due to incorporation of the thermoplastic elastomer. However, the resin composition still suffers from a reduction in impact resistance due to incorporation of a crystal nucleating agent. Accordingly, the impact resistance of resin composition is insufficient when the composition is used in the production of the piping member. Further, when an amount of the incorporated thermoplastic elastomer is increased in order to inhibit a reduction of the impact resistance, a problem of deteriorating a transparency of the piping member is caused.
In addition, since the above method is especially developed to produce the polypropylene sheets, the melt flow rate of the polypropylene homopolymer and ethylene-propylene random copolymer, before kneading thereof, is generally in the range of 1 to 10 g/10 min. Such a melt flow rate is considered to be excessively high, if the composition is used to form pipes using an extrusion molding process. Therefore, using the above method, molded pipes cannot be produced due to draw down of the composition during molding. Further, even if the molded pipes could be produced, the resulting pipes suffer from a shortened service life due to insufficient creep resistance of the pipes. Furthermore, since the impact strength is insufficient for the piping member, there is a possibility of generating problems such as breakage of the pipes.